reserve N for Nat;
reserve n,m,n1,n2 for Nat;
reserve q,r,r1,r2 for Real;
reserve x,y for set;
reserve w,w1,w2,g,g1,g2 for Point of TOP-REAL N;
reserve seq,seq1,seq2,seq3,seq9 for Real_Sequence of N;

theorem Th17:
  -(-seq) = seq
proof
  thus -(-seq)=(-1)*(-seq) by Th11
    .=(-1)*((-1)*seq) by Th11
    .=((-1)*(-1))*seq by Th13
    .=seq by Th16;
end;
